Group-III nitride semiconductors such as gallium nitride (GaN) have been conventionally used for fabricating a nitride semiconductor device such as light-emitting diode (LED) or laser diode (LD) (see, for example, Non-Patent Document 1). The Group-III nitride semiconductor is known to have a relatively wide bandgap at room temperature and, for example, the room-temperature band gap of gallium nitride and aluminum nitride (AlN) having a hexagonal wurtzite structure is as large as 3.4 eV and 5.9 eV, respectively (see, for example, Non-Patent Document 2). Therefore, the Group-III nitride semiconductor layer is used as a functional layer such as clad layer or light-emitting layer of a light-emitting device. The Group-III nitride semiconductor layer having such a large bandgap is advantageous in constituting a junction structure having high barrier. For example, a high mobility transistor comprising a heterojunction of electron supply layer and electron channel layer constituted by using an aluminum gallium nitride mixed crystal (AlXGa1−XN: 0<X≦1) having a bandgap of 3.4 eV or more is disclosed (see, for example, Non-Patent Document 3).
On the other hand, boron phosphide-base compound semiconductors such as boron monophosphide (BP) are known as an indirect Group III-V compound semiconductor.
Unlike Group-III nitride semiconductors such as gallium nitride (GaN), a p-type electrically conducting layer can be readily obtained by intentionally doping an impurity to the boron phosphide-based compound semiconductor. For example, a technique of doping magnesium (Mg) as a p-type impurity to obtain a p-type electrically conducting layer is disclosed (see, for example, Patent Document 1). Accordingly, it is expected that a pn-junction having a barrier difference can be readily obtained by joining a boron phosphide-based compound semiconductor layer having a wide bandgap and a Group-III nitride semiconductor layer.
Here, the room-temperature bandgap of, for example, boron phosphide has been traditionally known to be 2.0 eV (see, for example, Non-Patent Document 2) and in recent years, a technique to obtain a wider room-temperature bandgap from 2.8 to 3.4 eV by the optimization or the like of vapor growth conditions has been developed. However, in order to constitute heterojunction having a barrier height from a boron phosphide-based compound semiconductor layer and a Group-III nitride semiconductor layer such as gallium nitride, the traditional bandgap, 2 eV, given for borron phosphide is insufficient and a boron phosphide-based compound semiconductor layer having a wider bandgap is necessary. A boron phosphide-based compound semiconductor layer having a wide bandgap suitable for constituting heterojunction with a wide-bandgap semiconductor such as Group-III nitride semiconductor has been heretofore not reported.
(Patent Document 1)
JP-A-2-288388 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
(Non-Patent Document 1)
Isamu Akasaki (compiler), III Zoku Kagobutsu Handoutai (Group-III Compound Semiconductor), 1st ed., Chap. 13-14, Baifukan (Dec. 8, 1999)
(Non-Patent Document 2)
Isamu Akasaki (compiler), III-V Zoku Kagobutsu Handoutai (Group III-V Compound Semiconductor), 1st ed., page 150, Baifukan (May 20, 1994)
(Non-Patent Document 3)
Isamu Akasaki (compiler), III Zoku Kagobutsu Handoutai (Group-III Compound Semiconductor), 1st ed., Chap. 6-8, Baifukan (Dec. 8, 1999)